"Biodiversity" means "the variety of life in a particular habitat." Each life form, from bacterium to towering tree, plays a role in sustaining its ecosystem. Therefore, each life form matters.

There are few ecosystems on Earth with more biodiversity than a rainforest. A recent study shows that the loss of even a few species from that rainforest adversely affects its longevity. Although a forest may look healthy, if the creatures required for maintenance are missing, the forest can't regenerate if it's disturbed, and its trees will eventually die out.

Research from the Department of Life Sciences at Imperial College London, published in Proceedings of the Royal Society B, shows that intensive land use, such as for agriculture and ranching, often leads to the extinction of local forest birds. These birds perform "biodiversity services" that are necessary for maintaining the rainforest's health, such as keeping down the population of plant-eating insects and dispersing the seeds of tree species. But it isn't just the individual birds species that guarantee the future health of the forest, nor is it the number of species left alive; it's the way these species interact with other kinds of life.

The research team studied the composition of bird communities from 330 study sites in the Brazilian Amazon, sampling more than 450 bird species. They also kept track of what special traits these species possessed, such as their beak size and tail and wing shape. (These traits indicate what kind of job a species has evolved to do.) Then they looked at how landscape change affected these bird populations, specifically those birds who eat insects and those who disperse seeds.

The results were sobering. When insect-eating birds go locally extinct, leaf-eating insects can prevent young saplings from growing up into mature trees. When birds that eat certain seeds are missing, then the trees that grow from those seeds eventually go missing, too.

Dr Joseph Tobias, senior author of the study, says that land-use management policy can positively affect forest recovery. He suggests that a forest's ability to regenerate can be preserved, even if it's largely cleared, as long as patches of primary forest survive. “Our findings are a warning flag that we can’t just look at a snapshot of forest health as it appears now—we need to think about preserving the ecosystem processes that will allow forests to survive in the future.”

Next, the team plans to examine the impact of human activity on global ecosystems by using bird traits as a window onto the effects of environmental change.